Method for minimizing tension variation in a paper web induced by drying of the web in a paper machine

ABSTRACT

Tension variations in the paper web on a paper machine are caused by successive draws and moisture fluctuations. Good strength properties require optimization of the draws. In addition, the tension variation induced by moisture must be minimized. The analysis of local modulus and the strain components can be used for determining optimal locations for profiling actuators in order to achieve good tension profile. Longitudinal strain components of the web, i.e. reversible elastic strain, permanent plastic strain and frozen-in strain relievable with water are continuously measured and determined and the variation of said longitudinal components is minimized by correcting the moisture content of the paper web in a dryer section of the paper machine and/or by optimizing the draws between dryer groups in the dryer section.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a U.S. national stage application of InternationalApplication No. PCT/FI01/00855, filed Oct. 1, 2001, and claims priorityon Finnish Application No. 20002162, filed Sep. 29, 2000, the disclosureof each application being hereby incorporated by reference herein.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH AND DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

The present invention relates to drying-induced tension variation in apaper web. More specifically, the invention relates to a method forminimizing tension variation in a paper web induced by drying of the webin a paper machine.

On-machine coating and calendering processes integrated into a papermachine have set tight constraints on tension variation in the paperweb. A single weak spot in the paper web simultaneously with a tensionsurge can destroy the performance of the whole papermaking line. Inaddition, a process integrated into the machine does not include anydampening stage typical of conventional off-machine finishing processestaking place apart from the machine. In a machine roll, moisture andtemperature variations can even out. Also, local stress concentrationsare reduced during a long stress relaxation period.

The recent development of the wet press and dryer sections of the papermachine allows a higher web speed. The runnability of the paper machineis improved by closing all open draws and by using other new components.Due to this development, control of dryer section parameters has broughtnew potential. The magnitude of wet and dry straining levels can bevaried widely.

The objective of this paper is to examine how moisture variation andseveral successive draws affect the theological properties of paperduring the manufacturing process. At present, tension profile is themost important available on-line measurement technique for evaluatingthe theological properties of the paper web. Thus, special emphasis hasbeen placed on understanding the significance of development ofcross-direction tension profiles in a dry paper web.

In the literature, one can find several studies on the development offiber morphology and on the rheological properties of paper duringdrying. See: [1] Jentzen, C. A., The effect of stress applied duringdrying on some of the properties of individual pulp fibers. Tappi 47(1964) 7, s. 412-418. [2] Nanko, H., Asano, S. & Ohsawa, J., Shrinkingbehavior of pulp fibers during drying. Papers presented at 1991International Paper Physics Conference. Kona, Hawaii, Sep. 22-26, 1991.TAPPI Press, Atlanta, 1991. Book 2, s. 365-373. [3] Htun, M., Theinfluence of drying strategies on the mechanical properties of paper.Doctoral thesis, Royal Inst. Technol., Stockholm 1980, 31 s. [4] Page,D. H. & Tydeman, P. A., Physical processes occurring during the dryingphase. Transactions of the symposium held at Cambridge, September 1965,Vol. 1, Ed. F. Bolam, Tech. Sect. Br. Pap. Board Makers' Assoc., London,1966, s. 371-396. [5] Giertz, H. W. & Rodland, G., Elongation ofsegments—bonds in the secondary regime of the load/elongation curve.Tappi 1979 International paper physics conference. British Columbia,Sep. 17-19, 1979. Tech. Sect. Can. Pulp Pap. Assoc., Montreal, 1979, s.129-136. [6] Schulz, J. H., The Effect of Straining During Drying on theMechanical and Viscoelastic Behavior of Paper. Tappi, 44 (1961) 10, s.736-744.

Most studies have compared free drying, restraint drying and wetstraining. Also, the effect of removal of different water fractions fromfibers on shrinkage, hornification, wrinkling etc. is well established.See: [7] Weise, U., Characterization and mechanism of changes in woodpulp fibres caused by water removal. Doctoral thesis, HelsinkiUniversity of Technology, Department of forest products, Espoo 1997, 141s. [8] Berthold, J., Water adsorption and uptake in the fibre cell wallas affected by polar groups and structure. Doctoral thesis, Royal Inst.Technol., Stockholm 1996, 41s. [9] Laivins, G. V. & Scallan, A. M., Themechanism of hornification of wood pulps. Products of papermaking,Transactions of the tenth fundamental research symposium held at Oxford,September 1993, Vol. 2., Ed. C. F. Baker, Pira International, Surrey1993, s. 1235-1260. [10] Maloney.

It is well known that the elastic modulus of dry paper increases by theaction of wet straining, while breaking strain decreases at the sametime.

SUMMARY OF THE INVENTION

The present invention relates to a method for minimizing tensionvariation in a paper web induced by drying of the web in a papermachine. Tension variations in the paper web are caused by successivedraws and moisture fluctuations. Good strength properties requireoptimization of the draws. In addition, the tension variation induced bymoisture must be minimized. The analysis of local modulus and the straincomponents can be used for determining optimal locations for profilingactuators in order to achieve good tension profile. In the methodaccording to the invention, longitudinal strain components of the web,i.e. reversible elastic strain, permanent plastic strain and frozen-instrain relievable with water are continuously measured and determinedand the variation of said longitudinal components is minimized bycorrecting the moisture content of the paper web in a dryer section ofthe paper machine and/or by optimizing the draws between dryer groups inthe dryer section.

BRIEF DESCRIPTION OF THE DRAWINGS

Not applicable.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In this study, samples with different initial solids content weresubjected to several straining cycles in an ordinary tensile testingmachine during drying. Thus, a more paper-machine-like drying processwas obtained than in ordinary laboratory tests. Pilot machine trial runswere also performed to compare the results. In addition, water wasapplied to dry paper in order to find out the effect of drying historyon dimensional stability.

The tension profile in the paper machine is a combined effect of thelocal stiffness properties and the local strain in paper. Stiffness canbe characterized by local elastic modulus and, perhaps, by localrelaxation. Local strain can be divided into three strain componentsthat affect the tension profile of dry paper. Elastic strain recoverswhen drying or external stress is relieved. Plastic strain is permanentand probably caused by sliding of fibers in wet paper and by otherpermanent deformations in fiber wall structure. The third component,frozen-in strain, is relieved when dry paper is rewetted.

In restraint drying, initial moisture variation in wet sheet does notinduce differences in the elastic modulus or strain properties of paper.This means that the changes in the rheological properties of a paper webare caused by the combined effect of moisture variation and straining.The local modulus and the relative properties of the three straincomponents depend on the solids content at which the straining sequencesoccur. Thus, the initial moisture variation induces different localdrying histories. In the paper machine this effect is even amplified,because the moisture variation increases during the drying process.

It has been shown that tension variation is brought about by successivedraws and moisture fluctuations. The amplitude of draws causes the finaltension difference between two samples with different initial solidscontents. In our samples, a 3% initial moisture difference in the presssection induced a variation of up to 30% in tension.

Good strength properties require optimization of draws. In addition, thetension variation induced by moisture must be minimized. Analysis oflocal modulus and strain components can be used for determining optimallocations for profiling actuators in order to achieve a good tensionprofile.

As is already clear from the above description, an object of the presentinvention is to achieve minimal web tension variations. A further objectis to achieve as even a distribution of longitudinal components aspossible in the web. With a view to achieving these objects as well asthose coming out later, the invention is mainly characterized in thatlongitudinal strain components of the web, i.e. reversible elasticstrain, permanent plastic strain and frozen-in strain relievable withwater are continuously measured and determined in the method and thevariation of said longitudinal components is minimized by correcting themoisture content of the paper web in a dryer section of a paper machineand/or by optimizing the draws between dryer groups in the dryersection.

The mechanisms causing tension variations have been described above.Thus, the new papermaking concept in accordance with the invention ismainly based on the following things:

First of all, in the examination, by web tension is meant the product ofthe tensile strength and the elastic strain of the web, i.e. webtension=tensile strength*elastic strain.

The draws of the web during drying in the dryer section do not cause anysubstantial variation in the elastic modulus of dry paper. Thus, minimalweb tension variation is achieved when the variation of the longitudinalcomponents of paper have been minimized.

The longitudinal components of paper in turn refer to elastic strain,plastic strain and frozen-in strain which is relieved with water.

For adjustments, the longitudinal components of paper need to bemeasured. They are measured as follows:

Elastic strain=web tension/elastic modulus (Iqtension/STFI). Measurementof tension is carried out at the end of the dryer section of the papermachine at a location where the conditions of tension measurement do notchange.

Plastic strain=total strain—elastic strain. Total strain is determinedas an image analysis from a drying cylinder.

The frozen-in strain relievable with water is based on plastic strainand on a laboratory calibration curve in which the amount of theshrinkage potential of pulp and the amount of the frozen-in strainproduced by different draw levels have been measured.

The profiles of elastic strain and plastic strain determine the tensionvariation in a dry web. The frozen-in strain profile affects the tensionvariations in the web in connection with rewetting. Rewetting occurs,among other things, in surface sizing, coating and calendering.

As known, the dry solids content of the web increases in the dryersection of the paper machine in the running direction of the web, i.e.when the web moves forward in the dryer section. In the dryer section,attempts are made to keep the draws between dryer groups as short aspossible. However, at the beginning of the dryer section where the drysolids content is lowest and, on the other hand, at the end of the dryersection where the dry solids content is highest, the draws between thedryer groups are not the most critical from the point of view of tensionvariations. At a dry solids content of below 60% (in the beginning ofthe dryer section), local moisture faults in the web do not cause anysignificant profile errors when the web is subjected to draws. The paperis drawn to a sufficient extent to achieve the tensile strength/elasticmodulus of the paper.

In an attempt to achieve minimal tension variation and as uniform adistribution of the longitudinal components as possible in the web theseproperties can be affected in a number of ways in accordance with theinvention. In the method in accordance with the invention, the primarymode of operation used is that actuators correcting the moisture contentof the web are arranged in the area of the beginning of the dryersection where the dry solids content of the web is below 60%. Theseactuators can be drying and/or moisturizing actuators, which providedrying/moisturizing profiling, respectively. Impingement devices, wateratomizing/spraying devices and equivalent can be mentioned as examplesof drying/moisturizing actuators. The moisture-correcting actuators arecontrolled by measurement of the moisture content of the web. Themoisture content measurement is carried out most advantageously in thedryer section in the area where the dry solids content of the web is ina range of 75 to 85%. This area is the area with the highest moisturevariation and, when measurement is accomplished in this area, theresolution of measurement is the best possible.

The variation of longitudinal components caused by the local moisturefaults of the web when it is subjected to draws is highest in the areaof the dryer section where the dry solids content of the web is in arange of 60 to 85%. For this reason, in the method in accordance withthe invention, the draws are minimized in this area. In addition, thereis reason to use, for example, impingement units in this area becausethey can provide conditions corresponding to restraint drying (minimumdraw).

The profile of the frozen-in strain relievable with water can beinfluenced to some extent by the draws of the dryer section. The effectof draws on the web and on the frozen-in strain relievable with water isat a dry solids content of 55 to 65% opposite to that of a dry solidscontent of 75 to 85%. Because of this characteristic, in accordance withthe invention, it is possible to make the draws to influence in adesired direction at different locations of the dryer section. Thislongitudinal component profile can also be affected by using amoisturizing actuator when the dry solids content of the web is from 55to 65%.

By way of summary of the invention, it may be stated that with a view tominimizing tension variation in a paper web induced by drying of theweb, longitudinal strain components of the web, i.e. reversible elasticstrain, permanent plastic strain and frozen-in strain relieved withwater are continuously measured and determined in the method and thevariation of said longitudinal components is minimized by correcting themoisture content of the paper web in the dryer section of the papermachine and/or by optimizing the draws between dryer groups in the dryersection.

The moisture content of the paper web is advantageously affected in thearea of the beginning of the dryer section, when the dry solids contentof the web is below 60%, by means of actuators which correct themoisture content of the web and by which the web is moisturized and/ordried, and the operation of said moisture-correcting actuators iscontrolled by measuring the moisture content of the web in the dryersection in the running direction of the web after themoisture-correcting actuators. The moisture content of the web ismeasured in the dryer section preferably in the area where the drysolids content of the web is in a range of 75 to 85%.

In order to minimize the variation of the longitudinal strain componentsof the web caused by local moisture faults of the paper web when it issubjected to draws, the draws are most advantageously minimized in thedryer section in the area where the dry solids content of the web is ina range of 60 to 85%.

To affect the profile of the water-relievable frozen-in strain of thepaper web in one direction, the web is subjected to a draw or draws inthe area of the dryer section where the dry solids content of the web isin a range of 55 to 65% or, in a corresponding way, in the oppositedirection, in the area where the dry solids content of the web is in arange of 75 to 85%. To affect the profile of the frozen-in strainrelievable with water, it is also possible to use an actuator thatmoisturizes the web in the area of the dryer section where the drysolids content of the web is in a range of 55 to 65%.

To measure the elastic strain of the paper web, the tension of the webis measured in the end area of the dryer section. To measure plasticstrain, the total strain of the web is determined as an image analysisfrom a drying cylinder of the dryer section. The water-relievedfrozen-in strain of the paper web is determined based on plastic strainand a laboratory curve, which laboratory curve shows the measuredshrinkage potential of pulp and the measured amount of the frozen-instrain caused by different draw levels.

The different embodiments of the invention may vary within the inventiveidea defined in the accompanying claims.

1. A method for minimizing tension variation in a paper web induced bydrying of the web in a paper machine having a dryer section with dryergroups and draws between said dryer groups, the method comprising thesteps of: measuring and determining longitudinal strain components ofthe web, including reversible elastic strain, permanent plastic strainand frozen-in strain relievable with water; and minimizing variation ofsaid longitudinal strain components by correcting the moisture contentof the paper web in the dryer section of the paper machine and/or byoptimizing the draws between dryer groups in the dryer section, whereinin an area of the beginning of the dryer section, when the dry solidscontent of the web is below 60 percent, the moisture content of thepaper web is affected by means of actuators which correct the moisturecontent of the web and by which the web is moisturized and/or dried, andthat the operation of said moisture-correcting actuators is controlledby measuring the moisture content of the web in the dryer section in therunning direction of the web after the moisture-correcting actuators. 2.The method of claim 1, wherein the moisture content of the web ismeasured in the dryer section in the area where the dry solids contentof the web is in a range of 75 percent to 85 percent.
 3. The method ofclaim 1, wherein in order to minimize the variation of the longitudinalstrain components of the web caused by local moisture faults of thepaper web when it is subjected to draws, the draws are minimized in thedryer section in the area where the dry solids content of the web is ina range of 60 percent to 85 percent.
 4. A method for minimizing tensionvariation in a paper web induced by drying of the web in a paper machinehaving a dryer section with dryer groups and draws between said dryergroups, the method comprising the steps of: measuring and determininglongitudinal strain components of the web, including reversible elasticstrain, permanent plastic strain and frozen-in strain relievable withwater; and minimizing variation of said longitudinal strain componentsby correcting the moisture content of the paper web in the dryer sectionof the paper machine and/or by optimizing the draws between dryer groupsin the dryer section, wherein in order to affect the profile of thewater-relievable frozen-in strain of the paper web in one direction, theweb is subjected to a draw or draws in the area of the dryer sectionwhere the dry solids content of the web is in a range of 55 percent to65 percent or, in a corresponding way, in the opposite direction, in thearea where the dry solids content of the web is in a range of 75 percentto 85 percent.
 5. A method for minimizing tension variation in a paperweb induced by drying of the web in a paper machine having a dryersection with dryer groups and draws between said dryer groups, themethod comprising the steps of: measuring and determining longitudinalstrain components of the web, including reversible elastic strain,permanent plastic strain and frozen-in strain relievable with water; andminimizing variation of said longitudinal swain components by correctingthe moisture content of the paper web in the dryer section of the papermachine and/or by optimizing the draws between dryer groups in the dryersection, wherein in order to affect the profile of the water-relievablefrozen-in strain of the paper web, an actuator that moisturizes the webis used in the area of the dryer section where the dry solids content ofthe web is in a range of 55 percent to 65 percent.
 6. A method forminimizing tension variation in a paper web induced by drying of the webin a paper machine having a dryer section with dryer groups and drawsbetween said dryer groups, the method comprising the steps of: measuringand determining longitudinal strain components of the web, includingreversible elastic strain, permanent plastic strain and frozen-in strainrelievable with water; and minimizing variation of said longitudinalstrain components by correcting the moisture content of the paper web inthe dryer section of the paper machine and/or by optimizing the drawsbetween dryer groups in the dryer section, wherein in order to measurethe plastic strain of the paper web, the total strain of the web isdetermined as an image analysis from a drying cylinder of the dryersection.
 7. A method for minimizing tension variation in a paper webinduced by drying of the web in a paper machine having a dryer sectionwith dryer groups and draws between said dryer groups, the methodcomprising the steps of: measuring and determining longitudinal straincomponents of the web, including reversible elastic strain, permanentplastic strain and frozen-in strain relievable with water; andminimizing variation of said longitudinal strain components bycorrecting the moisture content of the paper web in the dryer section ofthe paper machine and/or by optimizing the draws between dryer groups inthe dryer section, wherein the water-relievable frozen-in strain of thepaper web is determined based on plastic strain and a laboratory curve,which laboratory curve shows the measured shrinkage potential of pulpand the measured amount of the frozen-in strain caused by different drawlevels.